Printers that include a thermal print head for printing information on a web of record members, such as labels, are known. One such type of printer is a thermal direct printer utilizing heat sensitive paper on which printing is directly carried out by the thermal print head. Such printers have been known to employ a thermal print head having a series of print elements aligned in a row wherein each element produces heat in response to energy applied thereto in order to print a dot. Such print elements may be resistors or the like to which a pulsed signal is applied to drive the print elements to print. In order to control the amount of energy applied to the print elements of a thermal print head, known systems have controlled the width or amplitude of the pulses applied thereto.
In another known system, a fixed number of pulses of a calculated duty cycle are applied to the thermal print head to print each row, wherein the duty cycle is calculated in response to measured, initial values of the thermal print head resistance and temperature as well as the input voltage applied to the thermal print head. In this system, the amount of energy applied to all print elements is made fixed so as to enable the printer to print narrow bars having a width as nominal as possible. This system works well for printing parallel bar codes as shown in FIG. 2 in which the length of the bars forming the code extends parallel to the direction of movement of the stock on which the information is printed. However, this system has several problems in printing serial bar codes in which the length of the bars forming the code extends perpendicular to the direction of movement of the stock as shown in FIG. 3. One such problem is that the serial bar code, and human readable characters if any, appear to be lighter when printed with this system. Further, in printing wide bars which are formed by printing narrow bars side by side, discontinuities such as a gap in the wide bar can result as shown in FIG. 4. These gaps result in wide bars that appear light. When human readable characters are printed adjacent to the serial code as shown in FIG. 5, discontinuities in the characters can result too. Such discontinuities include variations in the darkness of the dots forming portions of a character wherein lighter dots result in those portions of the character that are in a row adjacent to a space of the serial bar code, darker dots resulting in those portions of the character that are in a row adjacent to a bar of the serial bar code. In addition, it has been found that with this system the width of each bar printed increases across the length of the serial bar code or label. As information is being printed, each energized printing element loses heat to the area surrounding the element. This localized, short term temperature increase in the surrounding area results in succeeding information rows starting at a higher temperature than the initial temperature from which the duty cycle is calculated. Because succeeding rows start at a higher temperature, the width of dots printed in each succeeding row increases.